If it can't take AM3, is there any viable CPU upgrade available, or am I realistically going to have to replace both the mobo and the CPU? I can't afford both at present, so that will have to wait if so.

Looks like AM3+ is not supported on that board, AM3/AM2+/AM2 are supported but it depends on the on PCB version and bios version, so you need to check the PCB version printed on the motherboard (or on the label of the motherboard box) and possibly update the bios to the latest version.